Incorrect size calculations with fractional scaling under X11

**Describe the bug**
When using Tiling Shell under X11 with fractional scaling enabled (150% in my case), sizes are calculated incorrectly.
It seems that the scaling factor is messed up for some reason, probably because X11 actually doesn't support fractional scaling, so it scales resolution to an integer value and then just downscales it back. The layout logic itself remains correct, only the actual size calculations are affected.

**To Reproduce**
Steps to reproduce the behavior:
Start a GNOME session under X11.
Enable fractional scaling (e.g. 150%) in Display Settings.
Trigger tiling layout.
Notice that window dimensions don’t match the expected one.

**Screenshots**

> Incorrect sizing under X11 + fractional scaling (150%)

> Expected sizing

**Information:**
Tiling Shell version: v17.0
GNOME version: v48.0

**Additional context**
This issue doesn’t occur on Wayland.